This invention relates to a process for the purification of unsaturated hydrocarbons from a gaseous mixture containing hydrocarbons and relatively noncondensable gases including oxygen and CO.sub.2.
Unsaturated hydrocarbons such as styrene, butene and butadiene are commercially produced by the catalytic dehydrogenation of more saturated hydrocarbons. Butadiene is produced in large quantities by the dehydrogenation of butane and butene. Improved processes for the preparation of unsaturated hydrocarbons such as butenes, butadiene-1, 3, isoprene or styrene are processes wherein hydrocarbons such as butane, butene, isopentene, isopentane or ethylbenzene are dehydrogenated at elevated temperatures in the presence of catalysts and oxygen (halogen may also be present). Superior results and yields of products are thereby obtained. However, the product streams contain not only the desired unsaturated hydrocarbons but also may contain various by-products such as CO, CO.sub.2, hydrogen, nitrogen, oxygen, oxygenated hydrocarbons, acetylenic compounds, unreacted hydrocarbon, etc. When air is used as a source of oxygen, the effluent from the dehydrogenation reactor will contain large quantities of certain relatively noncondensable gases, such as nitrogen. The gaseous effluent will also contain varying amounts of steam.
Various problems exist in regard to the economic separation and purification of unsaturated hydrocarbons produced by oxidative dehydrogenation which are not encountered in the recovery of products produced by dehydrogenation in the absence of oxygen. Consequently, techniques utilized for the recovery and purification of products derived from the dehydrogenations in the absence of oxygen have not generally been found satisfactory for the recovery of effluents resulting from oxidative dehydrogenation reactions. The presence of large quantities of oxygen, by-products gases, and gases such as nitrogen create entirely different problems from those previously encountered. Furthermore, another problem encountered in the recovery of effluents from oxidative dehydrogenation reactors is that a high degree of fouling of recovery equipment is encountered. In view of these and other problems, a process was needed which would recover and purify the unsaturated hydrocarbon in an economical and efficient manner.
In U.S. Pat. Nos. 3,402,215 and 3,412,171, a process was disclosed whereby the unsaturated product may be recovered from the various gases present and from the acetylenic compounds at the same time. According to those processes, a particular gaseous mixture comprising unsaturated hydrocarbons, oxygen and inert noncondensable gases may be separated by intimately contacting the gaseous mixture in an absorbing zone with an oil having a boiling point or a boiling point range from about 170.degree. to 320.degree. F (at least 95 volume percent of the oil boils within this range). Acetylenic compounds were an impurity to which particular attention was given in these processes.
The absorbing oil, containing absorbed gases, including dehydrogenated material, unreacted feed, oxygen, inert noncondensable gases and steam or entrained water is cooled and is passed to separating zone where the predominate amount of oxygen and inert noncondensable gases and some acetylenic compounds are removed. In the present invention, this particular zone is not intended to achieve a separation of acetylenic compounds and their removal at this point is at most incidental to the removal of oxygen and inert noncondensable gases (CO, CO.sub.2 and nitrogen are considered to be the principal constituents of the inert noncondensable gases when air is fed to the oxidative dehydrogenation zone).
The optimal operation of the separating zone should achieve total removal of oxygen and the inert noncondensable gases without loss of the product unsaturated hydrocarbons or unreacted starting materials, e.g., butadiene and n-butene, respectively. In practice, of course, the separation is less than optimal, in that product unsaturated hydrocarbon and other desirable hydrocarbons are carried overhead with the stripped oxygen and inert noncondensable gases. The practice in the past, in the operation of the separator has been to carry some steam and entrained water overhead with the oxygen and inerts with the major portion passing out with the bottoms. The next step in the process is the removal of the hydrocarbons from the absorber oil. The hydrocarbons then go on to a final finishing operation and the lean oil is preferably recycled to the absorbing zone.
Conditions which allowed the water to be removed with the bottoms also resulted in less than the best separation of oxygen and CO.sub.2 from hydrocarbon product. This invention provides a more economical process, both in terms of cost and hydrocarbon product, since less hydrocarbon product passes overhead in the oxygen inert gas stream and better separation of oxygen and inert noncondensable gases from hydrocarbon product are achieved. Another benefit of the present invention is the substantial removal of water from the hydrocarbon product stream.
The present invention has as its goal the disclosure of a process which will provide an improved separation of hydrocarbon product from the oxygen and inert noncondensable gases overhead in a stripping zone and recovery of a liquid stream of water and a purer hydrocarbon product.